The invention relates to a method and a system for scanning terahertz waves. The system comprises a reflection device, a No.1 refraction device, a No.2 refraction device, a transmission device and a detection device, wherein the reflecting surface, the refracting surfaces or the transmitting surface of the devices are perpendicular to a common plane; the reflection device oscillates around an oscillating axle which is perpendicular to the common plane in the reflecting surface and reflects the terahertz waves onto the refracting surface of the No.1 refraction device; the No.1 refraction device and the No.2 refraction device rotate at angular velocities of the same size and opposite directions from the same initial position by taking a coincident center line of the two refraction devices as a rotation axis, wherein wedge angles theta 1 and theta 2, refractive indexes n1 and n2, and light path lengths L1 and L2 of the terahertz waves from an object point to the refracting surfaces of the No.1 refraction device and the No.2 refraction device meet the following relations that: L1*tan(arcsin(n1sin theta1)-theta1)=L2*tan(n2sin(arcsin(arcsin(n1sintheta1)-theta1)/n2+theta2))-theta2), the No.1 refraction device and the No.2 refraction device refract the terahertz waves onto the transmitting surface of the transmission device; and the transmission device converges the terahertz waves onto the detection device positioned at an image point. By the technical scheme, targets to be detected can be quickly scanned.